Neotropical forests are the greatest terrestrial reservoirs of biodiversity and carbon [1]. Despite prolonged attention of researchers to habitat degradation, forest fragmentation and deforestation [2], currently tropical forests are the most threatened ecosystems worldwide [3]. High rates of deforestation and selective logging in turn have resulted in a large increase in secondary forest areas [4]. The resilience of secondary forests is poorly understood, although Norden et al. [5] showed that secondary forests could serve as habitat refugia and biodiversity reservoirs of tree species from mature tropical forests.

A common problem with studies of secondary forest ecology and succession, however, has been uncertainty over stand age. Tropical secondary forest succession is a process that takes several decades, and to our knowledge, no continuous data on vegetation change are available for secondary forests over periods longer than 30 years. Consequently, the chronosequence approach, where disturbance ages of secondary forests are known, has been frequently used to study secondary forest dynamics [6]. Nevertheless, due to the difficulty of assessing the ages of secondary forest areas [7], there is a clear need for accurate and repeatable methodologies that allow us to date the age of disturbances over decadal time scales. More recently, attention has also been given to the need to monitoring and quantification of carbon sequestration processes to implement mechanisms for Reducing Emissions from Deforestation and Forest Degradation (REDD). However, one of the critical limitations in doing so is accurately estimating forests ages [8].

Currently, the most frequently used methods to date secondary forest are interviews of local people, use of remote-sensing techniques, probabilistic approaches based on tree growth rates, ¹⁴C dating, and tree ring analysis [7], [9]–[10]. All of these methods have limitations [9]. For example, information from local people is not always available, and its accuracy is highly variable [7]. Remote sensing or ¹⁴C dating are inaccurate and/or expensive techniques that may only work when suitable material is available.

Here, we demonstrate that Cecropia, a widespread and abundant pioneer genus in the Neotropics, is an accurate chronometer for dating different types of disturbance. We base this demonstration on a causal chain of previous observations, i.e. the Cecropia developmental periodicity allows predicting the age of individuals of this genus [11]–[12]. In secondary forests, the age of individuals of pioneer tree species reflects age of the forest and thereby age of the disturbance, since these species are incapable of germination following canopy closure [7]. In this study, we tested whether our estimation of Cecropia age based on two species and three different types of disturbance was reliable to the age of disturbance by studying the relationship between the estimated age of Cecropia and the real age of the disturbance. In addition, we propose twenty-one Cecropia species distributed all over the geographical range of the genus as potential chronometer species.

We found that Cecropia estimated age was highly correlated with the disturbance real age (F_1,31 = 701.8; P<0.001; Pearson's correlation: r² = 0.97, P<0.001; Fig. 2a), the slope is not different from 1 (slope = 1.04), and the intercept is different from zero (intercept = −0.26). This relationship was neither different between the two Cecropia species nor among the disturbance types (interaction P values≥0.152). Although Cecropia estimated age and real age of the disturbance were both square-root transformed to meet normality and homoscedasticity assumptions, we also present untransformed data in figure 2b. To determine a confidence interval for the correlation coefficient between estimated Cecropia age and disturbance real age we performed a non-parametric bootstrap. The size of the confidence interval was very small (95% CI: 0.965–0.976), indicating that the uncertainty in our correlation coefficient is small.

Based on the presence of 21 Cecropia species that show annual flowering [15], we generated a distribution map to highlight some of the potential Cecropia chronometers available across the Neotropics (Fig. 3 and Table 1).

The correlation coefficient of the relationship between Cecropia estimated age and disturbance real age was very close to one. Moreover, in a previous study Zalamea [14] showed that the production of new nodes is extraordinarily stable throughout plant ontogeny and among populations of C. sciadophylla, both within and among years. This stability was also observed despite a high variation in annual precipitation and a wide geographic range among the study sites, suggesting a strong genetic control and a weak climatic or environmental influence on the observed periodicity. Together, both results show the high potential of the genus Cecropia as a proxy to estimate the age of secondary forests in the Neotropics.

In the relationship between Cecropia estimated age and disturbance real age (Fig. 2), the intercept was negative and close to zero, suggesting a small lag in recruitment of Cecropia individuals after perturbation. In anthropogenic perturbations, such as slash-and-burn agriculture, mining exploitation, and road construction, soil is highly disturbed, and seed banks could be affected or eliminated. For example, fires lasting five hours, such as those commonly applied when clearing land for agriculture, are sufficient to eliminate the C. sciadophylla seed bank up to a depth of 15 cm (Zalamea & Stevenson unpublished data). Cecropia is common on disturbed areas affected by fire and mining, indicating that recruits may come from seed rain rather than soil seed bank. Thus, the small lag in recruitment could be a response by timing of disturbance relative to seed rain, yet this lag is no longer than a year. In Barro Colorado Island, Panama, most C. insignis germination occurred in natural gaps within the first year after gap opening [19], and in central Amazonia, C. sciadophylla and C. purpurascens form even-aged stands in gaps resulting from different land uses [20], suggesting that Cecropia recruitment occurs in a single pulse, early in the regeneration process. Although several factors could delay recruitment, the intercept close to zero show that this delay in our database, is short in time, usually a few months.

Our method is limited in applicability by the maximal longevity of Cecropia individuals. Assessing individual longevity in tropical trees is challenging [21]–[24], and in the case of Cecropia species, longevity data are scarce. In Cecropia, these estimations vary among 35 years for C. obtusifolia [9], 54 years for C. sciadophylla [25] and 96 years for C. insignis [26]. Although the oldest chronosequence used in this study was 20 years old, in a recent study we have dated a 34 years old C. sciadophylla individual in French Guiana [12]. Thus, at least for the first four decades after disturbance, the method described in this study provides very accurate estimations of secondary forest ages.

Although Cecropia is a widespread genus on the Neotropics, the method proposed here to estimate the age of disturbed areas could not be used yet for all the species of the genus. This is because we do not know how many nodes are produced per year for each Cecropia species. However, as flowering is one of the traits we used to estimate the age of Cecropia individuals in our previous studies [11]–[12], annual flowering periodicity suggests high potential for Cecropia species to become secondary forest chronometers, as it has been proposed for C. obtusa [11] and C. sciadophylla [12]. The distribution map of the annually flowering Cecropia species (Fig. 3) implies that the Cecropia dating method could be used across a wide geographical range (i.e. all the Neotropics). Preliminary results showed an annual growth periodicity for C. angustifolia, C. ficifolia, C. insignis and C. distachya (P.C. Zalamea and P. Heuret, unpublished data). These results reinforce the idea that Cecropia could be used as a chronometer to estimate the time since forest disturbance. Although this study was based in anthropogenic disturbances, this methodology could also be used to date natural gaps. However, the dense canopy occurring in natural gaps compared to anthropogenic disturbances complicates the Cecropia age estimation using binoculars (i.e. it is difficult to count all nodes along the main axis). In such cases, an accurate estimation could be made using a destructive method.

The age of pioneer trees not only provides information needed to calculate the recovery of carbon stocks that would help to improve forest management [8], but also provides information needed to characterize the initial floristic composition and the rates of species remigration into secondary forest [5], [7]. In addition, this information could be useful to establish differences in substrate or soil fertility among successional stands and to model secondary forest population dynamics [9], [22]. In the current Neotropical scenario of forest fragmentation and degradation, little is known about the variability of responses from different primary forest to degradation effects [5], [27]. Thus, our contribution shows how successional studies can be reliably and inexpensively extended without the need to obtain forest ages based on remote sensing data or potentially inaccurate interview data across the Neotropics. In addition, growth-monitoring studies in secondary forests in the Neotropics are scarce and recent, suggesting that integrative approaches that use chronosequences and permanent plots can provide the valuable knowledge urgently needed in the current portfolio of tools for secondary forests management.